This invention relates to a liquid crystal display device using a liquid crystal material having spontaneous polarization induced by application of an electric field or inherent thereto.
A liquid crystal display device has a feature of low voltage consumption, light in weight and the like, and are widely used for a display device of a word processor, personal computer or car navigation system. Particularly, a TN mode TFT-LCD having pixels connected to switching elements such as TFTs (thin film transistors) and using a nematic liquid crystal has an excellent display performance. However, the TN mode has a problem that the viewing angle is narrow and the response speed is low.
At present, a liquid crystal display element constructed by a liquid crystal material (antiferroelectric liquid crystal, ferroelectric liquid crystal or the like) having spontaneous polarization induced by application of the electric field or inherent thereto and held between two electrodes has received much attention as a display element having a wide viewing angle and high response speed.
Most types of liquid crystals having spontaneous polarization take three alignment states of no voltage application state, positive voltage application state and negative voltage application state.
Recently, liquid crystal materials such as a thresholdless antiferroelectric liquid crystal (TLAF), Deformed-Helix Ferroelectric liquid crystal (DHF), Twisted Ferroelectric liquid crystal (TFLC) or electric clinic, which could take an alignment state between the above three states according to an applied voltage in addition to the above three alignment states, were found among the liquid crystal materials having spontaneous polarization. The above liquid crystal materials have no or little memory characteristic, but a desired alignment state thereof can be held and gray scale display can be attained by using switching elements such as TFTs, TFDs (thin film diodes) or MIMs (metal-insulator-metal diodes) provided for the respective pixels in the active matrix system and holding the voltage during the non-selected period. As a result, a liquid crystal display device which can display gray scales with high speed and wide viewing angle can be attained.
The arrangement of molecules of a liquid crystal having spontaneous polarization is set in a state called a smectic phase. In the smectic phase, rod-like molecules are arranged in a layered form and set in parallel to one another as shown in FIGS. 1A and 1B.
If an external force is applied to the image plane of the liquid crystal display device by depressing the image plane by a finger, for example, the alignment of the liquid crystal is disturbed and the display becomes defective. In the TN mode or STN mode, since the liquid crystal has no layer structure, the alignment is naturally restored to the original state and the defective display can be cancelled when the external force is removed.
However, since the order parameter of the liquid crystal of the smectic phase is high, the disturbed layer structure cannot be restored even when the external force is removed if the alignment is once destroyed by application of the external force or the like. That is, the alignment of the liquid crystal is not restored to the original state and a portion to which the external force is applied remains semi-permanently as a display defective portion.
For example, in the case of antiferroelectric liquid crystal, if a force of 2 kg/cm2 or more is applied to the liquid crystal display element by a finger or the like, the layer structure of the smectic liquid crystal is disturbed as shown in FIGS. 1C and 1D and the alignment is not restored to the original state even if the force is removed, and an alignment defective region is formed.
Since the alignment degree of the liquid crystal molecules is lowered in the alignment defective region, display of black level is made poor (the transmission factor is high when black is displayed) and the contrast is lowered so that the display quality of the liquid crystal display device will be significantly degraded. Thus, the liquid crystal of smectic phase has a serious problem that the xe2x80x9calignment destructionxe2x80x9d occurs by finger-pressing or the like.
In order to restore the liquid crystal alignment which is once disturbed to a uniform state (to effect the alignment treatment), the following methods are provided.
(1) After the temperature of the liquid crystal is raised to a temperature of phase transfer to the isotropic phase or more, or a temperature approximately equal thereto, the temperature is gradually cooled to the room temperature.
(2) A relatively high AC voltage (generally, xc2x17V or more, preferably, xc2x110V or more is applied between the pixel electrode and the common electrode) which is approximately equal to the saturation voltage is applied to the liquid crystal (this method is also called a voltage application alignment treatment).
(3) A combination of the methods (1) and (2) is effected.
In the method (1), the liquid crystal display device can be carried into a electronic oven or the like and the liquid crystal can be easily heated to the phase-transfer temperature if the circuit is not yet mounted. However, if the TAB and driving circuit are mounted on the liquid crystal display device, the plastic-made casing and polarization plate are deformed or deteriorated when the whole portion of the liquid crystal display device is heated in the electronic oven and thus it is extremely difficult to heat the liquid crystal to the phase transfer temperature without giving any influence on other members.
Further, the method (1) is effective only when the liquid crystal molecules exhibit the nematic phase at a temperature higher than that for the smectic C-phase as in a certain type of DHF. However, the method is not effective when the liquid crystal makes phase transfer from the isotropic phase to the smectic phase without passing through the nematic phase as in the case of thresholdless antiferroelectric liquid crystal.
In the method (2), it is possible to apply a sufficiently high voltage by use of a function generator and amplifier if the driving circuit is not yet mounted on the liquid crystal display device. However, the inventors of the present invention studied this method and found that the following problems would occur if this method was applied to the liquid crystal display device having switching elements such as TFTs.
It is necessary to apply a voltage higher than the pixel voltage by approximately 15V or more in order to turn ON the TFT element. Therefore, in order to apply a high voltage to the pixel electrode and effect the alignment treatment, it is necessary to apply a gate voltage which is higher than usual. However, if the high voltage is applied to the gate, a problem that the reliability of the TFT element is lowered due to degradation of the insulating property of the gate insulating film has occurred.
Further, the characteristics of switching elements provided for the respective pixels slightly fluctuate and the fluctuation of the characteristic becomes significant when a voltage of xc2x15V or more is applied to the pixel electrode. When the voltage of xc2x15V or more is applied to the pixel electrode to effect the alignment treatment, the effective values of the applied voltages are slightly different depending on the respective pixels and the degree of the alignment treatment becomes different for each pixel, thereby making the display state worse.
If the TAB and driving circuit are mounted on the liquid crystal display device, only a maximum voltage of xc2x15V, can be applied to the pixel electrode, since the maximum amplitude of a withstand voltage of the normal driver IC is 5V, or the maximum amplitude is 10V when a special driver IC is used. Therefore, a problem that a high voltage (xc2x17V or more) necessary for the alignment treatment cannot be applied to the pixel electrode occurs.
Further, in a case wherein the gates are driven based on the line-at-a-time scanning method, the write time (in which one TFT is kept ON) is different depending on the definition of the image plane but is 10 to 70 xcexcs. If the response time of the liquid crystal is longer than the write time, the electric field response of the liquid crystal is not completed in the write period of time and the liquid crystal tends to make a response by consuming charges stored on the storage capacitor, so that the holding rate will be lowered and the effective voltage applied to the liquid crystal will be lowered. As a result, there occurs a problem that the sufficient alignment treatment cannot be effected for the liquid crystal.
An object of this invention is to provide a liquid crystal display device which can easily restore the liquid crystal alignment, even if the driving circuit and the like are mounted, and can always display an image of high contrast and good quality.
In order to attain the above object, a liquid crystal display device according to a first aspect of this invention comprises a first base plate; a plurality of pixel electrodes arranged in rows and columns on the first base plate; a plurality of switching transistors formed in correspondence to the plurality of pixel electrodes, each of the plurality of switching transistors having a gate electrode and a source and a drain region and one of the source and the drain region being connected to a corresponding one of the plurality of pixel electrodes; a plurality of scanning lines arranged on the rows of the first base plate, each of the plurality of scanning lines being connected to the gate electrode of a corresponding one of the plurality of switching transistors; a plurality of signal lines arranged on the columns of the first base plate, each of the plurality of signal lines being connected to the other of the source and the drain region of the corresponding one of the plurality of switching transistors; a second base plate arranged in opposition to a surface of the first base plate on which the plurality of pixel electrodes are formed; a common electrode arranged on the second base plate; a liquid crystal material sealed between the first and the second base plate and having spontaneous polarization; driving means for simultaneously selecting and driving a desired number of scanning lines among the plurality of scanning lines; and voltage applying means for applying a desired voltage to the common electrode.
It is preferable that the desired number of scanning lines are adjacent to one another and arranged side by side.
Further, it is preferable to set the desired number of scanning lines to 10 or more.
It is effective to use the scanning lines passing anyone of a portion in which the alignment of the liquid crystal material is disturbed and a portion in which image sticking is generated as the desired number of scanning lines.
The voltage applying means has an operation mode for applying the desired voltage in a non-display period.
The non-display period can be set as a period in which display is suspended for the sake of energy saving.
The voltage applying means applies a voltage for correcting anyone of alignment and image sticking of the liquid crystal material.
The voltage applying means can apply a signal voltage higher than a maximum value of a signal voltage, which is applied to the plurality of pixel electrodes, to the common electrode.
The voltage applying means can apply a voltage having a phase difference of 180xc2x0 with respect to a signal applied to a corresponding one of the plurality of pixels to the common electrode.
It is preferable to further comprise means for heating the liquid crystal material.
The heating means can contain the common electrode.
It is preferable to further comprise an external switching circuit for turning ON/OFF the driving means and the voltage applying means.
There is further provided writing means for writing positional information on a display image plane according to a mechanical stress applied from a surface of the second base plate which is farther away from the first base plate, and when the mechanical stress comes over a stress enough to disturb of the liquid crystal material, the writing means can generate a signal for operating the driving means and the voltage applying means.
According to this invention, ON signals are supplied to a plurality of scanning lines to turn ON the switching transistors connected to the scanning lines and a voltage is applied to the common electrode so as to restore the liquid crystal alignment even after the driving circuit and the like are mounted.
Further, by selecting a plurality of scanning lines, a sufficiently strong electric field can be stably and uniformly applied to liquid crystal molecules between the pixel electrode and the common electrode, thereby making it possible to easily restore the liquid crystal alignment.
A liquid crystal display device according to a second aspect of this invention comprises a first base plate; a storage capacitor electrode formed on the first base plate; an insulating film formed above the first base plate with the storage capacitor electrode disposed therebetween; a plurality of pixel electrodes arranged in rows and columns on the insulating film; a plurality of switching transistors formed in correspondence to the plurality of pixel electrodes on the insulating film, each of the plurality of switching transistors having a gate electrode and a source and a drain region and one of the source and the drain region being connected to a corresponding one of the plurality of pixel electrodes; a plurality of scanning lines arranged on the rows of the first base plate, each of the plurality of scanning lines being connected to the gate electrode of a corresponding one of the plurality of switching transistors; a plurality of signal lines arranged on the columns of the first base plate, each of the plurality of signal lines being connected to the other of the source and the drain region of the corresponding one of the plurality of switching transistors; a second base plate arranged in opposition to a surface of the first base plate on which the plurality of pixel electrodes are formed; a common electrode formed on the second base plate; a liquid crystal material sealed between the first and the second base plate and having spontaneous polarization; and voltage applying means for applying a desired voltage between the common electrode and the storage capacitor electrode.
It is preferable to further comprise means for simultaneously selecting and driving a desired number of scanning lines among the plurality of scanning lines.
The liquid crystal display device has an operation mode for setting a potential of the plurality of pixel electrodes into an electrically floating state when the voltage applying means applies the desired voltage between the common electrode and the storage capacitor electrode.
It is preferable that the display device further comprises a black matrix formed in correspondence to spaces between the plurality of pixel electrodes, and the storage capacitor electrode contains a portion formed in a region which faces the black matrix.
The voltage applying means applies the desired voltage in a non-display period.
The non-display period contains a period in which display is suspended for the sake of energy saving.
The voltage applying means can apply a voltage for correcting anyone of alignment and image sticking of the liquid crystal material.
The voltage applying means can apply a signal voltage higher than a maximum value of a voltage, which is applied to the plurality of pixel electrode, between the common electrode and the storage capacitor electrode.
It is preferable to further comprise means for heating the liquid crystal material.
The heating means can contain at least one of the storage capacitor electrode and the common electrode.
It is preferable to further comprise an external switching circuit for turning ON/OFF the voltage applying means.
In the first aspect of this invention, the alignment treatment by voltage application is effected by generating an electric field between the pixel electrode and the common electrode. Therefore, with the construction of the first aspect, the alignment treatment for the liquid crystal molecules on a region in which no pixel electrode is formed (that is, on the surrounding portion of the pixel electrode) cannot be effected. In order to prevent transmission of light through the non-pixel region, it becomes necessary to conceal the non-pixel region with a black matrix or the like. At the time of assembling the cell, since a margin of several xcexcm for alignment between the first base plate having pixel electrodes formed thereon and the second base plate having the black matrix formed thereon is required, it is necessary to make the black matrix thick and conceal part of the pixel electrodes by taking the alignment margin into consideration. As a result, the opening ratio is lowered. Further, when the alignment of the liquid crystal in the surrounding portion of the pixel is extremely low, the degree of the alignment of the liquid crystal of the pixel portion is influenced by the surrounding portion and is also lowered, thereby degrading the contrast.
Therefore, in the second aspect, means for applying a voltage between the common electrode and the storage capacitor electrode is provided to stably and uniformly apply a sufficiently strong electric field to liquid crystal molecules in the non-pixel region, thereby making it possible to restore the liquid crystal alignment.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore.